Pagerank, as we should come to know by now, is a measure of how authoritative a search engine determines a web page to be. Pagerank is usually on a scale of 0 to 10 with 10 being the highest.
Google provides a way for regular users to see the Pagerank of a page when using the Google toolbar. As we can imagine, the Pagerank (or any other statistical data from Google) shown will be somewhat out of date due to the sheer volume of traffic and processing that needs to be done on the data.
As a rough guide, Google updates the Pagerank as shown in the Google toolbar roughly quarterly, that is, every 3 months. These updates happen solely at Google's discretion so the 3 month figure is just a rough estimate to work with.
There is another form of Pagerank: the type that Google does not show via the toolbar. This is the most current value of Pagerank that Google has and it is used when determining the placements of websites on Search Engine Result Pages (SERPs). You can rest assured that this Pagerank is updated much more frequently than the one you will see in the toolbar.
As you can imagine, Google loves to present the most relevant results to users and as such, they will continually update this Pagerank figure as it is used in their internal processing.
The moral of this story is that you shouldn't worry too much about the Pagerank figure that the toolbar shows you unless you have a really old site. It your site is relatively new (less than a year old) and has dynamic bits, that figure will probably be outdated by the time you actually see it.
What you should do in the meantime is continue to publish quality unique content to keep Google and the other search engines (and other webmasters eager to link to something) coming back for more.